1. Field of the Invention
The invention relates to a raster scanning system and, more particularly, to a scanning system having a tangentially offset overfilled polygon ROS and spot size correction circuitry for modulating the pulse width of a drive signal for the source of radiant energy of the scanning system.
2. Discussion of the Related Art
Most conventional raster output scanners (ROS) utilize a rotating polygon having flat reflective surfaces, or facets, in parallel with the axis of rotation of the polygon. In a typical system, a light beam is emitted from a light source such as a helium-neon laser. The light beam is directed through pre-polygon conditioning optics, modulated according to an input electrical signal, and projected onto the rotating polygon surfaces. In these conventional ROS systems, the pre-polygon optics typically are incorporated in an underfilled facet design; e.g., the light beam directed against the rotating polygon illuminates only a portion of each rotating facet.
Overfilled facet designs, where the light beam at least partially illuminates more than one facet at all times, have been used to some degree but have not gained wide acceptance. Comparing the two designs, in an overfilled design, the facet size required to produce a given spot size at the image plane is greatly reduced, thus allowing many more facets to be accommodated on the same diameter polygon. Therefore, for a given polygon rotation speed, an overfilled polygon ROS will generate a higher scan line rate, and will thus be capable of running at higher process speeds. Increasing process speed capability is a significant advantage in that it enables the use of a single diode in an overfilled polygon ROS at significantly higher process speeds than in an underfilled polygon ROS.
Both overfilled and underfilled polygon ROS's, however, have an inherent problem in that sagittal offset of the pre-polygon beam introduces bow into the scan line. Although there are some other solutions to this problem, the most common solution is to use a tangentially offset design.
However, there exists a problem in using the tangentially offset overfilled design in that the spot size diameter in the direction of scan changes from the start of scan (SOS) position to the end of scan (EOS) position. This change in spot size occurs due to the change in the apparent width of the facet exposed to the light beam. The change in the apparent width of the facet results from the rotation of the polygon. In an overfilled design, since the facet defines the limiting aperture in the fast scan direction, the width of the spot at a photoreceptor plane is dependent on the apparent width of the facet. The variation in the spot size diameter in the fast scan direction can be as large as 20%. Such variation makes the tangentially offset overfilled design unacceptable for high quality ROS systems.